Just follow these steps:
Through the admin interface, move to the Import-Export option (1).
Type the data you want to import into the Import description text area (2) and click the Import data button (3).
Clicking the Sample data button (4) you will see the required fields in order to properly import your users.
These required fields are Login, First name, Last name & Email (5). You may add as many Course, Group and Branch columns as you like. When the login of a user already exists update his info. Make sure that Login is the first column. For a new user that does not have a password the login is used as a password. Active field gets as value YES or NO. Default is YES. Custom fields of the checkbox type get as value of "on" or "off".
If you already have the users in an excel file, you can directly copy and paste their data in the text box and import them. Please note that the excel column names should match the required fields, as described in the Sample data.